From my experience, planking is the best exercise to try to test your core strength. I used to train boxing full time for about 4 months and after our first workout we would always finish with a 5 or 6 minute abs session always finishing with the plank including side plank. Pick out 3 or 4 ab exercises such as crunches, elbow-to-knees etc. Do maybe 30 reps of each exercise then finish with a max effort plank and record how long you hold it for. Their is no easy and comfortable way of improve your core strength but it does only take 5 or 6 minutes out of your day to do.
